Polarization Mode Dispersion (PMD) is a phenomenon typical of optical fibers in which a slight birefringence in the means generates two different transmission states along the fiber. Its origin is normally due to the lack of circularity generated when manufacturing optical fibers, this effect increasing during the installation and wiring thereof. The delay between the two axes of polarization of the signal, produced by the difference in the effective refractive index observed by each polarization is defined as Differential Group Delay (DGD).
The PMD of a communication link can result in optical pulse spreading and deformation and in transmission errors, limiting the data rate of said link, so characterizing it is vital for determining the features of the link.
The evolution experienced by the polarization vector of a signal going through an optical fiber is wavelength-dependent and changing over time. The greater the PMD, the greater differentiation is observed in the State of Polarization (SOP) between two frequency components of a signal having the same polarization at its source, being able to be observed as a larger separation of the polarization vectors thereof in Stokes sphere.
The DGD can thus be obtained by splitting the angle φ formed by the polarization vectors of two spectral components of a signal between the difference in wavelength of said components. If the polarization vector of the frequency-resolved signal is referred to as S(ω), and the vector defined by the main states of birefringence of a system under analysis is referred to as Ω, the following is obtained:φ=sin−1{(S(ω1)×Ω)×(S(ω2)×Ω)/[|S(ω1)×Ω∥S(ω2)×Ω|]dS/dω=Ω×S DGD=φ/(ω2−ω1)
Ω is by definition the normal vector of the plane of rotation of the system, and it is unknown. In the case in which rotation occurs about a maximum circle, obtaining the angle φ is simplified to:φ=sin−1{(S1×S1)/(|S1∥S2|)}
In standard fiber systems the angle of rotation Ω is changing for each wavelength, and its influence on a signal going through the system is defined by the overall path that the polarization vector takes along the Stokes sphere.
The measurement of frequency-resolved SOP is therefore of great interest for characterizing PMD and DGD of optical communication links and systems.
Various methods and systems for taking said measurement are known. For example, EP 1,113,250 A1 discloses a method and system for determining the PMD of a device under test, in which a coherent light beam is split into two paths. The first beam goes through a polarization transformer and the device under test, and the second beam goes through a reference path. The superposition of the two resulting beams allows characterizing the Jones matrix of the device.
U.S. Pat. No. 6,563,590 B2 and U.S. Pat. No. 6,885,783 B2 disclose two examples based on optoelectronic heterodyne filters. Spectral measurements are taken for four states of a polarization transformer placed at the output of an internal laser of the heterodyne filter to characterize the signal.
In all cases, the precision of systems for measuring DGD is limited by the birefringence of the spectral filter, the conversion of the optoelectronic system, and the resolution of the filter, so there is a need in the state of the art for a system and method for measuring wavelength-resolved SOP with high precision and resolution.
Systems and methods with sufficient resolution for simultaneously measuring the SOP of several signals transported by a Dense Wavelength Division Multiplexing (DWDM) system with very small gaps between channels are particularly necessary. The resolution necessary for achieving this objective is unattainable with the devices known in the state of the art.